deepCopy(LogicalExpressionPlan lgExpPlan)
Create the deep copy of this expression and add that into the passed
LogicalExpressionPlan Return the copy of this expression with updated
logical expression plan.

getFieldSchema

Get the field schema for the output of this expression operator. This does
not merely return the field schema variable. If schema is not yet set, this
will attempt to construct it. Therefore it is abstract since each
operator will need to construct its field schema differently.